How big of a job would it be to change out the offending fabric in the blocks?
Most fabrics that have colored the water would eventually quit releasing color - when the rinse water was clear, I was fairly comfortable using the fabric.
HOWEVER, I've had some fabrics that bled and bled and bled and bled. After over 20 changes of water, the water was still fairly brightly colored.
Another HOWEVER - not all fabrics pick up stray dye - some seem to be resistant - and some aren't - maybe you could try soaking one of your blocks and see what happens?
I would not knowingly give a "problem child" to someone else.